The present invention concerns an improved apparatus for granulating and/or coating solid particles in a spouted bed.
Spouted beds are beds of particles obtained in an enclosure having a vertical axis and fitted at the lower part with a frustoconical part terminating at a bottom opening through which is introduced an upwardly directed gaseous flowstream. The upwardly directed gaseous flowstream maintains the particles in suspension and carries them along closed paths. Thus, the particles are directed upwardly by the gaseous flowstream in close proximity to the axis of the enclosure in the shape of a spout or geyser. The particles move radially toward the walls of the enclosure and then slowly return downward until they arrive at the top of the frustoconical part. Spouted beds of this type are described in French Pat. No. 1,125,461 filed on Mar. 11, 1955 and assigned to the National Research Council.
It is well known to use spouted beds for granulating fluid products which are hardenable either by evaporating water or another solvent or by cooling and/or for coating previously prepared particles or cores with these hardenable fluid products. According to the known granulating processes, a spouted bed of forming granules or of particles to be coated, is maintained by means of a hot or cold upwardly travelling gaseous flowstream with the hardenable product atomized in the gaseous flowstream beneath the spouted bed. The hardenable product is partly evaporated or cooled in the gaseous stream and deposits on the particles of the spouted bed located in the lower part of the frustoconical part. The deposited atomized product continues evaporating or cooling until solidification is obtained, while the particles are again carried upward by the gaseous flowstream within the axially directed spout, and while they return downwardly along the walls of the enclosure. When the particles again reach the lower part of the frustoconical part of the enclosure, they receive a new coating of the atomized product.
Granulation and/or coating is thus made by successively depositing the atomized hardenable product. Such a granulating process is described in U.S. Pat. No. 3,231,413 filed on Jan. 25, 1966 and is assigned to Potasse et Engrais Chimiques.
The apparatuses used up to now for granulating in a spouted bed include an enclosure having a vertical axis which has a cylindrically shaped part with a frustoconical part connected to the lower portion thereof.
For continuous operation, the apparatus includes a laterally extending overflow at the upper portion of the spouted bed in the cylindrical port for drawing granules off continuously. The gaseous stream is introduced through the small base of the frustoconical part of the enclosure. The spray nozzle for the coating product or for the product to be granulated is positioned in proximity to the frustoconical part of the enclosure.
It has been difficult to maintain a homogeneous bed with the prior art apparatuses, especially when using an apparatus having a diameter greater than, for example, 1 meter. As a result of maintaining a gaseous rate necessary for obtaining steady spouting conditions, large granules are often carried up to the cyclone dust collectors. The recycling ratio is therefore very high. Additionally, the bed is more likely to jam through the formation of granules which are too large.